All u have to do is make custom firmware
When U get phone download tiny umbrella and save the shsh files first and for most !!! Then make custom firmware in snow breeze (I faith) use 5.1.1 shsh file with the 5.1.1ipsw and u should be able to update and unlock again
See what baseband you're phone is on u can downgrade from 6.15 to 5.13.04
Message me if U need help